RNAase-III enzyme Dicer maintains signaling pathways for differentiation and survival in mouse cortical neural stem cells

MicroRNA cluster miR-17-92 regulates neural stem cell expansion and transition to intermediate progenitors in the developing mouse neocortex

During development of the embryonic neocortex, tightly regulated expansion of neural stem cells (NSCs) and their transition to intermediate progenitors (IPs) are critical for normal cortical formation and function. Molecular mechanisms that regulate NSC expansion and transition remain unclear. Here, we demonstrate that the microRNA (miRNA) miR-17-92 cluster is required for maintaining proper populations of cortical radial glial cells (RGCs) and IPs through repression of Pten and Tbr2 protein. Knockout of miR-17-92 and its paralogs specifically in the developing neocortex restricts NSC proliferation, suppresses RGC expansion, and promotes transition of RGCs to IPs. Moreover, Pten and Tbr2 protectors specifically block silencing activities of endogenous miR-17-92 and control proper numbers of RGCs and IPs in vivo. Our results demonstrate a critical role for miRNAs in promoting NSC proliferation and modulating the cell-fate decision of generating distinct neural progenitors in the developing neocortex.

Aging decreases antioxidant effects and increases lipid peroxidation in the Apolipoprotein E deficient mouse

In this study, to study the effect of aging and Apolipoprotein E (ApoE) deficiency on antioxidant ability in mice, we examined whether lipid peroxidation is promoted by aging in ApoE deficient (ApoE^−/−) mice, which have a shorter lifespan than normal mice. The levels of thiobarbituric acid-reactive substances (TBARS), a biomarker of lipid peroxidation, were measured in plasma and liver in ApoE^−/− mice aged 12 weeks (young) and 52 weeks (early stage of senescence). TBARS in plasma and liver were significantly increased by aging. Next, we examined the reasons why lipid peroxidation was promoted by aging, based on measurement of protein and mRNA levels for antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase) in liver in ApoE^−/− mice aged 12 and 52 weeks. The levels of superoxide dismutase 1 and 2 in liver were significantly decreased by aging. The mRNA level of catalase was also significantly decreased and the mRNA levels of superoxide dismutase 1, superoxide dismutase 2 and glutathione peroxidase 1 all showed a tendency to decrease with age. These results suggest that lipid peroxidation is caused by reduction of antioxidant activity with aging and that this promotes senescence and shortens lifespan in ApoE^−/− mice.

miR-130b is an EMT-related microRNA that targets DICER1 for aggression in endometrial cancer

Endometrial cancer (EC) is the most common gynecologic malignancy, but the molecular events involved in the development and progression of EC remain unclear. Certain microRNAs (miRNAs) and DICER1 play important roles in cell motility and survival. This study investigated the role of miR-130b and DICER1 in EC. We profiled miR-130b and DICER1 expression in clinical samples explored its relationship with clinical parameters. A luciferase reporter assay assessed the miR-130b targeting potential of DICER1. We show both in vitro and in vivo that miR-130b overexpression along with DICER1 dysfunction leads to tumor aggression and miRNA synthesis abnormalities that are related to cancer hallmarks through DICER1-miRNAs axis modulation. We also identify the mechanism related to this potential tumor predisposing phenotype: miR-130b and loss of DICER1 induced abnormal expression of EMT-related genes, which constitutes a loop regulation of the miR-130b-DICER1-EMT axis.

MicroRNAs: regulators of neuronal fate

Mammalian neural development has been traditionally studied in the context of evolutionarily conserved signaling pathways and neurogenic transcription factors. Recent studies suggest that microRNAs, a group of highly conserved noncoding regulatory small RNAs also play essential roles in neural development and neuronal function. A part of their action in the developing nervous system is to regulate subunit compositions of BAF complexes (ATP-dependent chromatin remodeling complexes), which appear to have dedicated functions during neural development. Intriguingly, ectopic expression of a set of brain-enriched microRNAs, miR-9/9* and miR-124 that promote the assembly of neuron-specific BAF complexes, converts the nonneuronal fate of human dermal fibroblasts towards postmitotic neurons, thereby revealing a previously unappreciated instructive role of these microRNAs. In addition to these global effects, accumulating evidence indicates that many microRNAs could also function locally, such as at the growth cone or at synapses modulating synaptic activity and neuronal connectivity. Here we discuss some of the recent findings about microRNAs' activity in regulating various developmental stages of neurons.

Approaches to manipulating microRNAs in neurogenesis

Neurogenesis in the nervous system is regulated by both protein coding genes and non-coding RNA molecules. microRNAs (miRNAs) are endogenous small non-coding RNAs and usually negatively regulate gene expression by binding to the 3′ untranslated region (3′UTR) of target messenger RNAs (mRNAs). miRNAs have been shown to play an essential role in neurogenesis, regulating neuronal proliferation, differentiation, maturation, and migration. An important strategy used to reveal miRNA function is the manipulation of their expression levels and patterns in specific regions and cell types in the nervous system. In this review we will systemically highlight established and new approaches used to achieve gain-of-function and loss-of-function of miRNAs in vitro and in vivo, and will also summarize miRNA delivery techniques. As the development of these leading edge techniques come online, more exciting discoveries of the roles miRNAs play in neural development and function will be uncovered.

A unilateral negative feedback loop between miR-200 microRNAs and Sox2/E2F3 controls neural progenitor cell-cycle exit and differentiation

MicroRNAs have emerged as key posttranscriptional regulators of gene expression during vertebrate development. We show that the miR-200 family plays a crucial role for the proper generation and survival of ventral neuronal populations in the murine midbrain/hindbrain region, including midbrain dopaminergic neurons, by directly targeting the pluripotency factor Sox2 and the cell-cycle regulator E2F3 in neural stem/progenitor cells. The lack of a negative regulation of Sox2 and E2F3 by miR-200 in conditional Dicer1 mutants (En1(+/Cre); Dicer1(flox/flox) mice) and after miR-200 knockdown in vitro leads to a strongly reduced cell-cycle exit and neuronal differentiation of ventral midbrain/hindbrain (vMH) neural progenitors, whereas the opposite effect is seen after miR-200 overexpression in primary vMH cells. Expression of miR-200 is in turn directly regulated by Sox2 and E2F3, thereby establishing a unilateral negative feedback loop required for the cell-cycle exit and neuronal differentiation of neural stem/progenitor cells. Our findings suggest that the posttranscriptional regulation of Sox2 and E2F3 by miR-200 family members might be a general mechanism to control the transition from a pluripotent/multipotent stem/progenitor cell to a postmitotic and more differentiated cell.

MicroRNAs and neuronal development

The importance of the involvement of non-protein coding RNAs in biological processes has become evident in recent years along with the identification of the transcriptional regulatory mechanisms that allow them to exert their roles. MicroRNAs (miRNAs) are a novel class of small non-coding RNA that regulates messenger RNA abundance. The capacity of each miRNA to target several transcripts suggests an ability to build a complex regulatory network for fine tuning gene expression; a mechanism by which they are thought to regulate cell fate, proliferation and identity. The brain expresses more distinct miRNAs than any other tissue in vertebrates and it presents an impressive variety of cell types, including many different classes of neurons. Here we review more than 10 years of miRNA research, and discuss the most important findings that have established miRNAs as key regulators of neuronal development.

Decoding the non-coding RNAs in Alzheimer's disease

Non-coding RNAs (ncRNAs) are integral components of biological networks with fundamental roles in regulating gene expression. They can integrate sequence information from the DNA code, epigenetic regulation and functions of multimeric protein complexes to potentially determine the epigenetic status and transcriptional network in any given cell. Humans potentially contain more ncRNAs than any other species, especially in the brain, where they may well play a significant role in human development and cognitive ability. This review discusses their emerging role in Alzheimer's disease (AD), a human pathological condition characterized by the progressive impairment of cognitive functions. We discuss the complexity of the ncRNA world and how this is reflected in the regulation of the amyloid precursor protein and Tau, two proteins with central functions in AD. By understanding this intricate regulatory network, there is hope for a better understanding of disease mechanisms and ultimately developing diagnostic and therapeutic tools.

MicroRNAs tune cerebral cortical neurogenesis

MicroRNAs (miRNAs) are non-coding RNAs that promote post-transcriptional silencing of genes involved in a wide range of developmental and pathological processes. It is estimated that most protein-coding genes harbor miRNA recognition sequences in their 3' untranslated region and are thus putative targets. While functions of miRNAs have been extensively characterized in various tissues, their multiple contributions to cerebral cortical development are just beginning to be unveiled. This review aims to outline the evidence collected to date demonstrating a role for miRNAs in cerebral corticogenesis with a particular emphasis on pathways that control the birth and maturation of functional excitatory projection neurons.

Role of microRNAs 99b, 181a, and 181b in the differentiation of human embryonic stem cells to vascular endothelial cells

MicroRNAs (miRNAs) are short noncoding RNAs, which post-transcriptionally regulate gene expression. miRNAs are transcribed as precursors and matured to active forms by a series of enzymes, including Dicer. miRNAs are important in governing cell differentiation, development, and disease. We have recently developed a feeder- and serum-free protocol for direct derivation of endothelial cells (ECs) from human embryonic stem cells (hESCs) and provided evidence of increases in angiogenesis-associated miRNAs (miR-126 and -210) during the process. However, the functional role of miRNAs in hESC differentiation to vascular EC remains to be fully interrogated. Here, we show that the reduction of miRNA maturation induced by Dicer knockdown suppressed hES-EC differentiation. A miRNA microarray was performed to quantify hES-EC miRNA profiles during defined stages of endothelial differentiation. miR-99b, -181a, and -181b were identified as increasing in a time- and differentiation-dependent manner to peak in mature hESC-ECs and adult ECs. Augmentation of miR-99b, -181a, and -181b levels by lentiviral-mediated transfer potentiated the mRNA and protein expression of EC-specific markers, Pecam1 and VE Cadherin, increased nitric oxide production, and improved hES-EC-induced therapeutic neovascularization in vivo. Conversely, knockdown did not impact endothelial differentiation. Our results suggest that miR-99b, -181a, and -181b comprise a component of an endothelial-miRNA signature and are capable of potentiating EC differentiation from pluripotent hESCs

Emerging role of non-coding RNA in neural plasticity, cognitive function, and neuropsychiatric disorders

Non-coding RNAs (ncRNAs) have emerged as critical regulators of transcription, epigenetic processes, and gene silencing, which make them ideal candidates for insight into molecular evolution and a better understanding of the molecular pathways of neuropsychiatric disease. Here, we provide an overview of the current state of knowledge regarding various classes of ncRNAs and their role in neural plasticity and cognitive function, and highlight the potential contribution they may make to the development of a variety of neuropsychiatric disorders, including schizophrenia, addiction, and fear-related anxiety disorders.

Dysregulation of uterine signaling pathways in progesterone receptor-Cre knockout of dicer

Epithelial-stromal interactions in the uterus are required for normal uterine functions such as pregnancy, and multiple signaling pathways are essential for this process. Although Dicer and microRNA (miRNA) have been implicated in several reproductive processes, the specific roles of Dicer and miRNA in uterine development are not known. To address the roles of miRNA in the regulation of key uterine pathways, we generated a conditional knockout of Dicer in the postnatal uterine epithelium and stroma using progesterone receptor-Cre. These Dicer conditional knockout females are sterile with small uteri, which demonstrate significant defects, including absence of glandular epithelium and enhanced stromal apoptosis, beginning at approximately postnatal d 15, with coincident expression of Cre and deletion of Dicer. Specific miRNA (miR-181c, -200b, -101, let-7d) were down-regulated and corresponding predicted proapoptotic target genes (Bcl2l11, Aldh1a3) were up-regulated, reflecting the apoptotic phenomenon. Although these mice had normal serum hormone levels, critical uterine signaling pathways, including progesterone-responsive genes, Indian hedgehog signaling, and the Wnt/β-catenin canonical pathway, were dysregulated at the mRNA level. Importantly, uterine stromal cell proliferation in response to progesterone was absent, whereas uterine epithelial cell proliferation in response to estradiol was maintained in adult uteri. These data implicate Dicer and appropriate miRNA expression as essential players in the regulation of multiple uterine signaling pathways required for uterine development and appropriate function.

Drosha regulates neurogenesis by controlling neurogenin 2 expression independent of microRNAs

Temporal regulation of embryonic neurogenesis is controlled by hypostable transcription factors. The mechanism of the process is unclear. Here we show that the RNase III Drosha and DGCR8 (also known as Pasha), key components of the microRNA (miRNA) microprocessor, have important functions in mouse neurogenesis. Loss of microprocessor in forebrain neural progenitors resulted in a loss of stem cell character and precocious differentiation whereas Dicer deficiency did not. Drosha negatively regulated expression of the transcription factors Neurogenin 2 (Ngn2) and NeuroD1 whereas forced Ngn2 expression phenocopied the loss of Drosha. Neurog2 mRNA contains evolutionarily conserved hairpins with similarities to pri-miRNAs, and associates with the microprocessor in neural progenitors. We uncovered a Drosha-dependent destabilization of Neurog2 mRNAs consistent with microprocessor cleavage at hairpins. Our findings implicate direct and miRNA-independent destabilization of proneural mRNAs by the microprocessor, which facilitates neural stem cell (NSC) maintenance by blocking accumulation of differentiation and determination factors.

Dynamic Roles of microRNAs in Neurogenesis

MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression at the post-transcriptional level by mediating mRNA degradation or translational inhibition. MiRNAs are implicated in many biological functions, including neurogenesis. It has been shown that miRNAs regulate multiple steps of neurogenesis, from neural stem cell proliferation to neuronal differentiation and maturation. MiRNAs execute their functions in a dynamic and context-dependent manner by targeting diverse downstream target genes, from transcriptional factors to epigenetic regulators. Identifying context-specific target genes is instrumental for understanding the roles that miRNAs play in neurogenesis. This review summarizes our current state of knowledge on the dynamic roles that miRNAs play in neural stem cells and neurogenesis.

Epigenetic control on cell fate choice in neural stem cells

Derived from neural stem cells (NSCs) and progenitor cells originated from the neuroectoderm, the nervous system presents an unprecedented degree of cellular diversity, interwoven to ensure correct connections for propagating information and responding to environmental cues. NSCs and progenitor cells must integrate cell-intrinsic programs and environmental cues to achieve production of appropriate types of neurons and glia at appropriate times and places during development. These developmental dynamics are reflected in changes in gene expression, which is regulated by transcription factors and at the epigenetic level. From early commitment of neural lineage to functional plasticity in terminal differentiated neurons, epigenetic regulation is involved in every step of neural development. Here we focus on the recent advance in our understanding of epigenetic regulation on orderly generation of diverse neural cell types in the mammalian nervous system, an important aspect of neural development and regenerative medicine.

MicroRNAs in Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder and is the most common form of dementia in the elderly. Accumulating evidence in AD research suggests that alterations in the microRNA (miRNA) network could contribute to risk for the disease. miRNAs are conserved small non-coding RNAs that control gene expression at the posttranscriptional level and are essential for neuronal function and survival. The results from recent profiling experiments in humans suggest that a number of specific miRNAs are misregulated in disease conditions, several of which have been implicated in the regulation of key genes involved in AD, including APP, BACE1 and MAPT. Moreover, rare disease-specific polymorphisms have been identified in known and putative miRNA target sites located within the 3'untranslated regions (3'UTRs) of APP and BACE1 genes. Here, we review current findings regarding miRNA research in humans and various cellular and animal models to provide a strong basis for future research aimed at understanding the potential contribution of miRNAs to AD pathophysiology.

Deletion of the RNaseIII enzyme dicer in thyroid follicular cells causes hypothyroidism with signs of neoplastic alterations

Micro-RNAs (miRNAs) are small non-coding RNAs that regulate gene expression, mainly at mRNA post-transcriptional level. Functional maturation of most miRNAs requires processing of the primary transcript by Dicer, an RNaseIII-type enzyme. To date, the importance of miRNA function for normal organogenesis has been demonstrated in several mouse models of tissue-specific Dicer inactivation. However, the role of miRNAs in thyroid development has not yet been addressed. For the present study, we generated mouse models in which Dicer expression has been inactivated at two different stages of thyroid development in thyroid follicular cells. Regardless of the time of Dicer invalidation, the early stages of thyroid organogenesis, preceding folliculogenesis, were unaffected by the loss of small RNAs, with a bilobate gland in place. Nevertheless, Dicer mutant mice were severely hypothyroid and died soon after weaning unless they were substituted with T4. A conspicuous follicular disorganization was observed in Dicer mutant thyroids together with a strong down regulation of Nis expression. With increasing age, the thyroid tissue showed characteristics of neoplastic alterations as suggested by a marked proliferation of follicular cells and an ongoing de-differentiation in the center of the thyroid gland, with a loss of Pax8, FoxE1, Nis and Tpo expression. Together, our data show that loss of miRNA maturation due to Dicer inactivation severely disturbs functional thyroid differentiation. This suggests that miRNAs are mandatory to fine-tune the expression of thyroid specific genes and to maintain thyroid tissue homeostasis.

MicroRNAs link estrogen receptor alpha status and Dicer levels in breast cancer

To identify microRNAs (miRNAs) associated with estrogen receptor (ESR1) status, we profiled luminal A, ESR1+ breast cancer cell lines versus triple negative (TN), which lack ERα, progesterone receptor and Her2/neu. Although two thirds of the differentially expressed miRNAs are higher in ESR1+ breast cancer cells, some miRNAs, such as miR-222/221 and miR-29a, are dramatically higher in ESR1- cells (∼100- and 16-fold higher, respectively). MiR-222/221 (which target ESR1 itself) and miR-29a are predicted to target the 3' UTR of Dicer1. Addition of these miRNAs to ESR1+ cells reduces Dicer protein, whereas antagonizing miR-222 in ESR1- cells increases Dicer protein. We demonstrate via luciferase reporter assays that these miRNAs directly target the Dicer1 3' UTR. In contrast, miR-200c, which promotes an epithelial phenotype, is 58-fold higher in the more well-differentiated ERα+ cells, and restoration of miR-200c to ERα- cells causes increased Dicer protein, resulting in increased levels of other mature miRNAs typically low in ESR1- cells. Together, our findings explain why Dicer is low in ERα negative breast cancers, since such cells express high miR-221/222 and miR-29a levels (which repress Dicer) and low miR-200c (which positively affect Dicer levels). Furthermore, we find that miR-7, which is more abundant in ERα+ cells and is estrogen regulated, targets growth factor receptors and signaling intermediates such as EGFR, IGF1R, and IRS-2. In summary, miRNAs differentially expressed in ERα+ versus ERα- breast cancers actively control some of the most distinguishing characteristics of the luminal A and TN subtypes, such as ERα itself, Dicer, and growth factor receptor levels.

Timing specific requirement of microRNA function is essential for embryonic and postnatal hippocampal development

The adult hippocampus consists of the dentate gyrus (DG) and the CA1, CA2 and CA3 regions and is essential for learning and memory functions. During embryonic development, hippocampal neurons are derived from hippocampal neuroepithelial cells and dentate granular progenitors. The molecular mechanisms that control hippocampal progenitor proliferation and differentiation are not well understood. Here we show that noncoding microRNAs (miRNAs) are essential for early hippocampal development in mice. Conditionally ablating the RNAase III enzyme Dicer at different embryonic time points utilizing three Cre mouse lines causes abnormal hippocampal morphology and affects the number of hippocampal progenitors due to altered proliferation and increased apoptosis. Lack of miRNAs at earlier stages causes early differentiation of hippocampal neurons, in particular in the CA1 and DG regions. Lack of miRNAs at a later stage specifically affects neuronal production in the CA3 region. Our results reveal a timing requirement of miRNAs for the formation of specific hippocampal regions, with the CA1 and DG developmentally hindered by an early loss of miRNAs and the CA3 region to a late loss of miRNAs. Collectively, our studies indicate the importance of the Dicer-mediated miRNA pathway in hippocampal development and functions.

Functions of noncoding RNAs in neural development and neurological diseases

The development of the central nervous system (CNS) relies on precisely orchestrated gene expression regulation. Dysregulation of both genetic and environmental factors can affect proper CNS development and results in neurological diseases. Recent studies have shown that similar to protein coding genes, noncoding RNA molecules have a significant impact on normal CNS development and on causes and progression of human neurological disorders. In this review, we have highlighted discoveries of functions of noncoding RNAs, in particular microRNAs and long noncoding RNAs, in neural development and neurological diseases. Emerging evidence has shown that microRNAs play an essential role in many aspects of neural development, such as proliferation of neural stem cells and progenitors, neuronal differentiation, maturation, and synaptogenesis. Misregulation of microRNAs is associated with some mental disorders and neurodegeneration diseases. In addition, long noncoding RNAs are found to play a role in neural development by regulating the expression of protein coding genes. Therefore, examining noncoding RNA-mediated gene regulations has revealed novel mechanisms of neural development and provided new insights into the etiology of human neurological diseases.

Functional dicer is necessary for appropriate specification of radial glia during early development of mouse telencephalon

Early telencephalic development involves transformation of neuroepithelial stem cells into radial glia, which are themselves neuronal progenitors, around the time when the tissue begins to generate postmitotic neurons. To achieve this transformation, radial precursors express a specific combination of proteins. We investigate the hypothesis that micro RNAs regulate the ability of the early telencephalic progenitors to establish radial glia. We ablate functional Dicer, which is required for the generation of mature micro RNAs, by conditionally mutating the Dicer1 gene in the early embryonic telencephalon and analyse the molecular specification of radial glia as well as their progeny, namely postmitotic neurons and basal progenitors. Conditional mutation of Dicer1 from the telencephalon at around embryonic day 8 does not prevent morphological development of radial glia, but their expression of Nestin, Sox9, and ErbB2 is abnormally low. The population of basal progenitors, which are generated by the radial glia, is disorganised and expanded in Dicer1^-/- dorsal telencephalon. While the proportion of cells expressing markers of postmitotic neurons is unchanged, their laminar organisation in the telencephalic wall is disrupted suggesting a defect in radial glial guided migration. We found that the laminar disruption could not be accounted for by a reduction of the population of Cajal Retzius neurons. Together, our data suggest novel roles for micro RNAs during early development of progenitor cells in the embryonic telencephalon.

MicroRNAs and Alzheimer's Disease Mouse Models: Current Insights and Future Research Avenues

Evidence from clinical trials as well as from studies performed in animal models suggest that both amyloid and tau pathologies function in concert with other factors to cause the severe neurodegeneration and dementia in Alzheimer's disease (AD) patients. Accumulating data in the literature suggest that microRNAs (miRNAs) could be such factors. These conserved, small nonprotein-coding RNAs are essential for neuronal function and survival and have been implicated in the regulation of key genes involved in genetic and sporadic AD. The study of miRNA changes in AD mouse models provides an appealing approach to address the cause-consequence relationship between miRNA dysfunction and AD pathology in humans. Mouse models also provide attractive tools to validate miRNA targets in vivo and provide unique platforms to study the role of specific miRNA-dependent gene pathways in disease. Finally, mouse models may be exploited for miRNA diagnostics in the fight against AD.

microRNAs in neurons: manifold regulatory roles at the synapse

The regulation of synapse formation and plasticity in the developing and adult brain underlies a complex interplay of intrinsic genetic programs and extrinsic factors. Recent research identified microRNAs (miRNAs), a class of small non-coding RNAs, as a new functional layer in this regulatory network. Within only a few years, a network of synaptic miRNAs and their target genes has been extensively characterized, highlighting the importance of this mechanism for synapse development and physiology. Very recent data further provide insight into activity-dependent regulation of miRNAs, thereby connecting miRNAs with adaptive processes of neural circuits. First direct links between miRNA dysfunction and synaptic pathologies are emerging, raising the interest in these molecules as potential biomarkers and therapeutic targets in neurological disorders.

MicroRNA function in the nervous system

MicroRNAs (miRNAs) are an extensive class of small noncoding RNAs that control posttranscriptional gene expression. miRNAs are highly expressed in neurons where they play key roles during neuronal differentiation, synaptogenesis, and plasticity. It is also becoming increasingly evident that miRNAs have a profound impact on higher cognitive functions and are involved in the etiology of several neurological diseases and disorders. In this chapter, we summarize our current knowledge of miRNA functions during neuronal development, physiology, and dysfunction.

Reversible block of mouse neural stem cell differentiation in the absence of dicer and microRNAs

Background

To investigate the functions of Dicer and microRNAs in neural stem (NS) cell self-renewal and neurogenesis, we established neural stem cell lines from the embryonic mouse Dicer-null cerebral cortex, producing neural stem cell lines that lacked all microRNAs.

Principal Findings

Dicer-null NS cells underwent normal self-renewal and could be maintained in vitro indefinitely, but had subtly altered cell cycle kinetics and abnormal heterochromatin organisation. In the absence of all microRNAs, Dicer-null NS cells were incapable of generating either glial or neuronal progeny and exhibited a marked dependency on exogenous EGF for survival. Dicer-null NS cells assumed complex differences in mRNA and protein expression under self-renewing conditions, upregulating transcripts indicative of self-renewing NS cells and expressing genes characteristic of differentiating neurons and glia. Underlining the growth-factor dependency of Dicer-null NS cells, many regulators of apoptosis were enriched in expression in these cells. Dicer-null NS cells initiate some of the same gene expression changes as wild-type cells under astrocyte differentiating conditions, but also show aberrant expression of large sets of genes and ultimately fail to complete the differentiation programme. Acute replacement of Dicer restored their ability to differentiate to both neurons and glia.

Conclusions

The block in differentiation due to loss of Dicer and microRNAs is reversible and the significantly altered phenotype of Dicer-null NS cells does not constitute a permanent transformation. We conclude that Dicer and microRNAs function in this system to maintain the neural stem cell phenotype and to facilitate the completion of differentiation.

Suppressor of fused and Spop regulate the stability, processing and function of Gli2 and Gli3 full-length activators but not their repressors

Gli2 and Gli3 are primary transcriptional regulators that mediate hedgehog (Hh) signaling. Mechanisms that stabilize and destabilize Gli2 and Gli3 are essential for the proteins to promptly respond to Hh signaling or to be inactivated following the activation. In this study, we show that loss of suppressor of fused (Sufu; an inhibitory effector for Gli proteins) results in destabilization of Gli2 and Gli3 full-length activators but not of their C-terminally processed repressors, whereas overexpression of Sufu stabilizes them. By contrast, RNAi knockdown of Spop (a substrate-binding adaptor for the cullin3-based ubiquitin E3 ligase) in Sufu mutant mouse embryonic fibroblasts (MEFs) can restore the levels of Gli2 and Gli3 full-length proteins, but not those of their repressors, whereas introducing Sufu into the MEFs stabilizes Gli2 and Gli3 full-length proteins and rescues Gli3 processing. Consistent with these findings, forced Spop expression promotes Gli2 and Gli3 degradation and Gli3 processing. The functions of Sufu and Spop oppose each other through their competitive binding to the N- and C-terminal regions of Gli3 or the C-terminal region of Gli2. More importantly, the Gli3 repressor expressed by a Gli3 mutant allele (Gli3(Delta699)) can mostly rescue the ventralized neural tube phenotypes of Sufu mutant embryos, indicating that the Gli3 repressor can function independently of Sufu. Our study provides a new insight into the regulation of Gli2 and Gli3 stability and processing by Sufu and Spop, and reveals the unexpected Sufu-independent Gli3 repressor function.